1. Field of the Invention
The present invention relates to a heat developable light sensitive material and more particularly, it is concerned with a heat developable light sensitive material which can not only be used to provide an image of a pure black tone but also show an improved shelf-life (which means the ability of the heat developable light sensitive material to retain those initial photographic characteristics possessed even after prolonged storage) and an increased speed of development.
2. Description of the Prior Art
Photography which uses silver halide has been carried out to a much greater extent, as compared with other photographic methods such as diazo photography, electrophotography and the like, because the former possesses superior photographic characteristics such as sensitivity, gradation, etc., than the latter. However, silver halide photography is time-consuming and requires a large amount of labor because several steps for processing the silver halide light sensitive material used therein are required; that is to say, the silver halide light sensitive material must be first subjected to an image wise-exposure and then, to a developmental processing with a developing solution, followed by stopping, fixation, washing and/or stabilization treatments with respective processing solutions therefor for the purposes of preventing of a rapid color change or a deterioration from occurring in the developed image area under the exposure to normal room illumination and of a blackening occurring in the non-developed white area (which is called background hereinafter). In addition, agents used therein adversely affect the human body when handled and further, rooms used for the above described processings, and the worker's hands and clothes as well, are stained with the agents used. Furthermore, the waste processing solutions, if discarded directly into a river, cause pollution problems. Therefore, it has been strongly desired to develop a method capable of processing highly sensitive silver halide photographic materials under dry conditions instead of the conventionally employed wet conditions using processing solutions, and capable of providing a processed image which is stable and that capable of reducing to a marked extent the color change occurring in the processed background through exposure to normal room illumination.
Many efforts have been made to solve these problems. For example, the addition of a 3-pyrazolidone type developing agent to a silver halide emulsion made it possible for the emulsion to be developed through the application of heat, as disclosed in German Patent Application (OLS) Nos. 1,123,203 and 1,174,157.
In addition, the combined addition of the above-described developing agent and a certain fixing agent suitable for silver halide to a silver halide emulsion has been proposed in German Patent Application (OLS) No. 1,003,578. However, it is impossible using these techniques to completely stabilize silver halide itself, which remains in the light sensitive material processed under dry conditions, against exposure to normal room illumination. Namely, a fixing process using dry conditions was not described in the above-described three German Patent Applications and further, in the method described in German Patent Application (OLS) 1,003,578, the developing agent (reducing agent) and the fixing agent present in the emulsion undergo an undesirable reaction upon storage, as would be expected. Therefore, these techniques appear to be rather impractical industrially.
In the art of making light sensitive materials capable of producig photographic images according to the dry process of the kind described above, the most successful photographic materials are the heat developable light sensitive materials as disclosed in U.S. Pat. Nos. 3,152,904 and 3,457,075; which utilize a composition containing, as essential components, the silver salt of an organic acid, a small amount of silver halide and a reducing agent. In this light sensitive system, the silver halide remaining in the light sensitive material after completion of development processing is allowed to simply change color through exposure to light without stabilizing it to light. Nevertheless, this system exhibits a satisfactory effect equivalent to other systems wherein the residual silver halide is subjected to a certain stabilizing treatment to light, since only a small amount of silver halide is incorporated therein and because, a large portion of the silver component is present in the form of a white or a pale yellow-colored organic silver salt which is so stable to light that it hardly blackens through exposure to light, and even if coloration does occur through a decomposition of the small amount of residual silver halide resulting from the light-exposure, the system is, on the whole, white or slighty colored light yellow and therefore, such a slight coloration is difficult to perceive by the human eye. The above-described light sensitive system can be used to produce an image according to the following mechanism where the system is heated to about 80.degree. C and preferably, to 100.degree. C, after the conclusion of image-exposure, the organic silver salt, which can act as an oxidizing agent, and the reducing agent (which are incorporated together into a light sensitive layer) undergo a redox reaction due to the inherent catalytic action of the small amount of exposed silver halide present in the vicinity thereof to result in a liberation of silver which quickly blackens the exposed area of the light sensitive layer and causes a clear contrast to the unexposed area thereof (background) to produce an image, although the system is inactive at ordinary temperatures (e.g., below about 70.degree. C).
As another example of a heat developable light sensitive materials of this kind, mention may be made of those which are disclosed in Japanese Patent Application (OPI) No. 4728/71, U.S. Pat. No. 3,933,507 and Japanese Patent Publication No. 25498/74, wherein light sensitive complex salts prepared from silver salts and dyes are employed as a photo-catalyst instead of the above-described silver halides. Furthermore, the combined use of a highly light sensitive organic silver salt and a rather low light sensitive organic silver salt can provide desirable effects upon heat developable light sensitive materials, as disclosed in Japanese Patent Application (OPI) No. 8522/75.
Most of the above-described heat developable light sensitive materials which utilize organic silver salts as oxidizing agents only provide yellowish brown colored images. However, they can be modified by the addition of an appropriate color toning agent thereto to result in the production of images of a good black tone. In U.S. Pat. No. 3,457,075, phthalazinone is used independently as a color toning agent. However, the independent use of phthalazinone has the disadvantage of adversely affecting the shelf-life and particularly, under the condition of high humidity. Namely, a heat developable light sensitive material which has been stored a long time in an atmosphere of a humidity higher than about 80% R.H. and then is exposed to light and developed through the application of heat results in the production of a brown-colored image and also, in a reduction in the maximum density of the resulting image and in the sensitivity of the resulting material.
Moreover, a phthalazinone-containing heat developable light sensitive material has another disadvantage, since phthalazinone tends to sublime to a great extent and therefore, the processing device used becomes contaminated with sublimed phthalazinone as it is used repeatedly in order to process a number of sheets of a phthalazinone-containing heat developable light sensitive material. With the intention of preventing phthalazinone from subliming on the occasion of heat development, the use of phthalazinone derivatives of the kind which contain substituent groups at some positions of the condensed benzene ring instead of using an un-substituted phthalazinone is disclosed in Japanese Patent Application (OPI) No. 67132/75 (corresponding to U.S. Patent Application Ser. No. 515,375, filed Oct. 16,1974). Heat developable light sensitive materials utilizing the above-described phthalazinone derivatives have excellent sublimation prevention properties and shelf-life under ordinary conditions of storage, but have some problems to be improved which concern the shelf-life under high temperature and humidity conditions. Further, heat developable light sensitive materials containing other phthalazinone derivatives which have carbamoyl groups at the N-positions thereof are disclosed in U.S. Pat. No. 3,844,797, but they have the disadvantage of low developing speed.
On the other hand, examples of compounds which are known as a color toning agent, other than the above-described phthalazinones, are cyclic imides including substituted phthalimides, as disclosed in British Pat. No. 1,380,795; quinazolinones and pyrazoline-5-ones, as disclosed in U.S. Pat. No. 3,846,136; N-hydroxynaphthalimides, as disclosed in U.S. Pat. No. 3,782,941; mercapto compounds as disclosed in U.S. Pat. No. 3,832,186 and U.S. Pat. No. 3,881,938; phthalazinediones as disclosed in Japanese Patent Application (OPI) No. 116471/73 (corresponding to U.S. Patent Application Ser. No. 515,375, filed Oct. 16,1974); uracils, barbituric acid, saccharin, 5-nitrosaccharin, phthalic anhydride, sulfolane, 2-mercapto- benzoxazole, 2-hydroxybenzothiazole, 2-amino-6-methylbenzothiazole, 2-amino-4-(4-biphenyl)thiazole, imidazole, 2-hydroxybenzimidazole and N,N' -ethylenethiourea, as disclosed in British Pat. No. 1,462,016; and so on. However, these known color toning agents provide less satisfactory results, since the color toning function inherent therein is insufficient, they adversely affect the shelf-lie or they reduce the speed of heat development.
Further, benzoxazinediones and quinazolidinediones as disclosed in U.S. Pat. No. 3,951,660 can be also employed as a color toning agent, but they have the same disadvantages as the above-described phthalazinones have when they are used independently; that is to say, when they are used sufficient shelf-life under high humidity conditions is not obtained to cause a deterioration in the color tone of the image, a reduction in the maximum color density and the large decrease sensitivity. Furthermore, the benzoxazinediones and the quinazolidinediones have another disadvantage in that the developing speed attainable with a light sensitive material containing one of these compounds independently just after the preparation thereof is inferior to that of a light sensitive material containing a phthalazinone independently.
In addition, the combined use of a benzoxazinedione or of a quinazolidinedione with a commonly used toner compound (e.g., an unsubstituted phthalazinone, a phthalazinone derivative, phthalimide or a phthalimide derivative) is disclosed in U.S. Pat. No. 3,951,660. The disclosure therein is that the concentration of an assistant toner compound is preferably lower than that of the benzoxazinedione or quinazolidinedione, but the effects resulting from this combined use are not at all described.
As is apparent from the above descriptions, it has been difficult to achieve simultaneously the production of images of a pure black tone and other properties; that is to say, a prevention of sublimation of a color toning agent from occurring, an improvement in the shelf-life of a heat developable light sensitive material under high humidity conditions and an improvement in the heat developing speed of a heat developable light sensitive material.